1. HURRICANES 



The Origin of Atlantic Hurricanes 



Atlantic hurricanes are uncommon 

 events by comparison with the fre- 

 quency of the storms that parade 

 across the temperate latitudes of the 

 United States each month. In terms 

 of their deadly, destructive potential, 

 however, they are, individually, the 

 greatest storms on earth and the 

 most important natural phenomena 

 to affect the welfare of the United 

 States. A single event may visit more 

 than a billion dollars of damage and 

 result in hundreds of lives lost, 

 mainly due to drownings. In addition 

 to carrying sustained winds that 

 sometimes exceed 150 miles per hour 

 and hurricane tides that may rise 

 more than 20 feet above mean sea 

 level, this massive atmospheric storm 

 often carries with it families of tor- 

 nadoes running ahead of the highest 

 tides and strongest winds. For ex- 

 ample, in Hurricane Beulah, which 

 moved into the lower Texas coast 

 in 1967, a total of 49 verified tor- 

 nadoes was reported. 



The quest for a better understand- 

 ing of the hurricane, for means of 

 increasing the accuracy of forecasts, 

 and, ultimately, for reducing the ex- 

 tent of hazard has focused attention 

 on the source regions of the seedling 

 disturbances from which hurricanes 

 grow, and on the environmental 

 structure of the equatorial trough and 

 the trade winds which control the 

 forces promoting hurricane develop- 

 ment. This quest has been greatly 

 assisted by a new tool of observa- 

 tion, the meteorological satellite, 

 which maintains a continuous global 

 surveillance of cloud groups produced 

 by disturbances and storm systems. 



Surveillance and Prediction of 

 Hurricane Seedlings 



On average, more than 100 hur- 

 ricane seedlings move westward 



across the tropical Atlantic during 

 the course of a hurricane season, 

 June through November. These seed- 

 lings are initially benign rain storms 

 which move westward in the flow of 

 the trade winds. Less than one-fourth 

 of the seedlings develop circulation 

 eddies with discrete centers of low 

 pressure, and an average of only 10 

 per year intensify enough to sustain 

 gale-force winds and earn a girl's 

 name as a tropical storm. On aver- 

 age, 6 of the 10 tropical storms reach 

 hurricane intensity at some stage in 

 their lifetime and 2 of these cross 

 the U.S. coastline. 



For many years, meteorologists 

 have known that some hurricanes 

 seem to have their origin near the 

 west coast of Africa. Not until the 

 meteorological satellite provided daily 

 information on storm systems around 

 the globe, however, was it apparent 

 not only that hurricane seedlings 

 could be traced back to the African 

 coastline in many instances, but also 

 that they seemed to stem from a 

 source region near the Abyssinian 

 Plateau of eastern Africa. They 

 march in great numbers across arid 

 portions of Africa before reaching the 

 Atlantic Ocean, where they begin ab- 

 sorbing the moisture necessary to 

 drive a vigorous storm system. 



A census of the hurricane seedlings 

 that occurred in 1968 is presented in 

 Figure V-l, which diagrams the 

 sources and movement of disturb- 

 ances and their evolution into tropi- 

 cal storms. The parade of disturb- 

 ances, mainly from Africa westward 

 across the Atlantic and Caribbean, 

 extends across Central America into 

 the eastern Pacific. Approximately 

 three-fourths of the eastern Pacific 

 storms are spawned by seedlings 

 whose origin is on the Atlantic side 

 of Central America. 



It is noteworthy, however, that 

 not all hurricanes form from seed- 

 lings which had sources in Africa. 

 Indeed, not all hurricanes form in the 

 tropics. Almost every year one or 

 two hurricanes develop from tem- 

 perate-latitude systems. Typically, the 

 trailing edge of an old worn-out cold 

 front, while losing its temperature 

 contrast, acquires a rich influx of 

 moisture from the tropics. The proc- 

 ess causes a circular storm to form 

 and to develop the structural char- 

 acter of the hurricane. Since this 

 process frequently takes place in close 

 proximity to a U.S. coastline, it poses 

 a particularly challenging warning 

 problem. 



Surveillance — The surveillance of 

 hurricane seedlings and of hybrid 

 disturbances that may become hurri- 

 canes is done mainly by satellite 

 cloud photography. Figure V-2, for 

 example, shows a series of hurricane 

 seedlings in the tropical Atlantic and 

 a hurricane that is lashing the Texas 

 coast — in this case, Hurricane Beu- 

 lah, September 18, 1967. Two tropi- 

 cal storms are also visible in the 

 eastern Pacific Ocean. In such photo- 

 graphs, the satellite looks down es- 

 sentially on the exhaust product of 

 the heat engine that generates the 

 clouds. 



At present, inferences about the 

 efficiency of the engine and the en- 

 ergy that is being released must be 

 drawn empirically and indirectly. 

 However, second-generation satellites, 

 and techniques for analyzing the 

 movement of cloud segments from 

 successive pictures, will soon provide 

 more direct means of assessing the 

 changes in horsepower that the heat 

 engine develops. 



The tracking and prediction of hur- 

 ricanes cannot be done with meteoro- 



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